This invention relates to apparatus for holding nuclear fuel assemblies and particularly to nuclear fuel assembly storage racks.
In nuclear steam supply systems well known in the art, a reactor vessel contains fuel assemblies with nuclear fuel therein which produce heat in a commonly understood fashion. The fuel assemblies may be rectangular or hexagonal arrays of fuel elements and may be approximately 150 inches in length. The fuel element may be a hollow cylindrical steel rod filled with nuclear fuel pellets as is well understood by those skilled in the art. When the fuel assemblies are placed in proper configuration within the reactor vessel, the fuel elements comprising the fuel assembly, generate heat. A coolant is circulated through the reactor vessel in heat transfer relationship with the fuel assemblies thereby transferring heat from the fuel assemblies to the coolant. The coolant may then be circulated to a location remote from the reactor vessel to generate steam and in turn generate electricity. After a period of reactor operation, the nuclear fuel in the fuel element becomes depleted necessitating replacement of the spent fuel assembly with a fresh one. The spent or depleted fuel assembly is then transferred to a storage location where it is allowed to cool to a reasonable temperature. Before and after being used in the reactor, the fuel assembly may be held upright in a storage location by a storage rack.
There are several storage rack configurations known in the art. In most of these storage racks the fuel assembly is held upright by having its upper end clamped to a supporting structure while having its lower end clamped or fitted into a socket. The fuel assemblies are separated by a sufficient distance to avoid a critical arrangement. While the clamped top end and clamped or socketed bottom end configurations provide positive retention structures, if the fuel assembly becomes slightly misaligned between clamps the configuration may result in high stresses in the fuel assembly because the configuration approximates a column with a clamped top and bottom. Thus, the slenderness of the fuel assembly and its substantial weight, which may be 1500 pounds, combined with a slight misalignment of the fuel assembly between clamps can result in excessive torsional and bending stresses in the fuel assembly. Since in many applications, these high stresses cannot be tolerated, the prior art configurations for storage racks are not suitable.